"Follow Track" motion tracking constraint depth to empty
I'll answer my own question... My original understanding was that the 'depth object' was an object that was purely used to indicate the distance from the 'camera' that the tracking the object would be placed. However, I've just stumbled on the realisation that it's much, much cleverer than that.... the 'depth object' is used to position the object on its surface at the point that it would intersect with the depth object - so a wavey mesh would position it at varying depth depending on which bit of the wave it happened to hit. This means that an empty (being a point) would be a completely pointless (excuse the pun) thing to use - hence it's not valid to use in that situation! If you want to have the track placed at a constant distance from the source then you could use a plane perpendicular to the camera; but if you want it to follow a more complicated path the the 'depth object' can be tailored to suit whatever profile you require.